Avionics Navigation Systems, 2nd Edition

2 The Navigation Equations

2.1 INTRODUCTION

The navigation equations describe how the sensor outputs are processed in the on board computer in order to calculate the position, velocity, and attitude of the aircraft. The navigation equations contain instructions and data and are part of the airborne software that also includes moding, display drivers, failure detection, and an operating system, for example. The instructions and invariant data are usually stored in a read-only memory (ROM) at the time of manufacturing. Mission-dependent data (e.g., waypoints) are either loaded from a cockpit keyboard or from a cartridge, sometimes called a data-entry device, into random-access memory (RAM). Waypoints are often precomputed in a ground-based dispatch or mission-planning computer and transferred to the flight computers.

Figure 2.1 is the block diagram of an aircraft navigation system. The system utilizes three types of sensor information (as explained in Chapter 1):

1. Absolute position data from radio aids, radar checkpoints, and satellites (based on range or differential range measurements).
2. Dead-reckoning data, obtained from inertial, Doppler, or air-data sensors, as a means of extrapolating present position. A heading reference is required in order to resolve the measured velocities into the computational coordinates.
3. Line-of-sight directions to stars, which measure a combination of position and attitude errors (as explained in Chapters 1 and 12).

The navigation computer combines ...